In my own experience, I'm no longer experiencing acetaldehyde flavours in my beer after improving my aeration methods and at the same time starting to use yeast nutrient.

In his IPA book, Mitch Steele says that the biggest cause by far of acetaldehyde comes from dying yeast cells from which acetaldehyde escapes. It's also stated that acetaldehyde production increases with yeast cell growth - so aeration will increase acetaldehyde production.

I only use a primary fermentor, and it is always used for two week fermentations.

So, my theory to explain why I'm experiencing less acetaldehyde is that because I aerate better and provide a better environment to the yeast with yeast nutrient, there are fewer yeast cells dying at the end of fermentation.

Does this sound plausible?

Since aeration promotes acetaldehyde production, I think this also means I should be careful when dry-hopping to introduce as little air as possible to the fermentor. Yes, it will be 'scrubbed' out by the yeast consuming it, but this also means the yeast are producing acetaldehyde. I wasn't worried about introducing air when dry hopping before - because the common concern is oxidation, but it seems to me we should also be concerned about acetaldehyde when dry hopping.

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Upvote on the question, and someone will undoubtedly come by with a better answer, but here goes off the top of my head:

Acetaldehyde (a-cee-tal-de-hide....nobody says it right!) is a precursor to alcohol. It is an intermediate compound that is formed prior to the formation of EtOH/ethanol during fermentation.

So the weird thing is that acetaldehyde is formed as a normal part of fermentation...but then it basically becomes the booze and is gone (hence in a well-fermented beer, you can't taste it). When EtOH is OXIDIZED, it will become acetaldehyde again, and then likely acetic acid, which is perceived as the cidery/sliced green apple flavor.

Gordon Strong says (this may be somewhat akin to what Mitch says):
"It is noticed mostly in young beers where the yeast were not able to reabsorb or finish the conversion of glucose to pyruvic acid to acetaldehyde and finally to ethanol. In other words, the beer was removed from the yeast too quickly."

As far as what Mitch says, yes, acetaldehyde will be produced more vigorously with aeration prior to pitching, HOWEVER, it will also lead to increased cell growth (and health), which will convert that acetaldehyde to EtOH and you will lose any gross green apple flavors.

In short, in my experience, acetaldehyde comes from low pitching rate into bigger beers, taking off the yeast too quickly (2-3 weeks minimum), or in some cases, too warm of a ferment temp (I know that the typical wisdom is that this leads to fusels, but I think any time you are creating a hostile environment for yeast, it can lead to a host of issues).

the only way I could see dry hopping causing increased perception of acetaldehyde is if the oxygen introduced when one violently throws his hop bag into the fermenter and the O2 dissolvesbonds to the EtOH and reverts it back to acetaldehyde, but I do not know if that is a concern at homebrewing hopping levels/production size. Also, if you are dry-hopping in the primary after fermentation, there should still be a decent CO2 blanket above the level of the beer.